This invention generally relates to an improved method for removing sludge and corrosion products from the interior of a heat exchanger vessel, which may be a nuclear steam generator, wherein a series of pressure pulses is generated within a liquid chemical cleaning agent after it has been introduced into the vessel to enhance chemical cleaning by dislodging, dissolving or fluidizing sludge and corrosion products within the vessel.
Methods for chemically cleaning the interior of heat exchanger vessels such as nuclear steam generators are known in the prior art. However, before the purpose and operation of such cleaning methods may be understood, some basic understanding of the structure and maintenance problems associated with nuclear steam generators is necessary.
Nuclear steam generators generally comprise a bowl-shaped primary side through which hot, radioactive water from the reactor core is circulated, a secondary side disposed on top of the primary side into which non-radioactive water is fed, and a tubesheet which includes a number of U-shaped heat exchanger tubes disposed between the primary and secondary sides of the generator for thermally connecting but hydraulically insulating the primary and secondary sides so that heat from the radioactively contaminated water in the primary side will be conducted to the non-radioactive water in the secondary side, thereby causing it to boil and to create non-radioactive steam.
The U-shaped heat exchanger tubes are contained within the secondary side of such steam generators. Each such heat exchanger tube is inverted, with its open ends mounted in the tubesheet and its legs and bent portion extending into the secondary side. A plurality of spaced apart support plates are provided in the secondary side for laterally supporting the legs of each heat exchanger tube. The legs of the U-shaped heat exchanger tubes extend through bores present in these support plates. Small, annular spaces are present between these heat exchanger tubes and the bores in the support plates and tubesheet through which these tubes extend. These annular spaces which are known in the art as "crevice regions". Such crevice regions provide only a very limited flow path for the feed water that circulates throughout the secondary side of the steam generator, which in turn can cause the feed water to boil so rapidly in these regions that they can actually dry out. This chronic drying-out causes impurities in the water to plate out and collect in these crevice regions. These impurities may promote the occurrence of sludges and corrosion on the exterior surfaces of the tubes, and the resulting corrosion products can, over time, accumulate in the crevice to the point where they can actually dent the heat exchanger tubes. Even though the heat exchanger tubes are made from corrosion-resistant Inconel 600 or 690, if the resulting impurities and sludges are not removed, sufficient corrosion and stress can occur in the crevice region areas of these tubes to ultimately cause them to crack unless some sort of maintenance operation is undertaken. Since a cracked heat exchanger tube can cause radioactive materials from the primary side to contaminate the non-radioactive water in the secondary side of the generator, it is important that maintenance operations are implemented which prevent such corrosion and subsequent cracking from occurring.
Chemical cleaning methods were developed in the prior art to dissolve such sludge accumulation, and to ameliorate corrosion. In these methods, the nuclear steam generator is first taken out of service and completely drained of water from both the primary and the secondary sides. Next, as most of the corrosion products contained within the crevice regions are iron oxide and copper that have become tightly ensconced in the crevice regions or on the surfaces of heat exchanger tubes, chelate-containing iron removal solvents and copper removal solvents are sequentially introduced into the interior of the secondary side to dissolve and remove them. Such iron removal solvents typically include an aqueous mixture of EDTA (ethylenediaminetetraacetic acid), hydrazine, ammonium hydroxide (NH.sub.4 OH), and an inhibitor for retarding corrosion reactions between the metal surfaces in the generator and the EDTA, known in the trade as CCI-801 available from Petrolite, Inc., located in St. Louis, Missouri. The copper solvent likewise includes an aqueous mixture of EDTA and NH.sub.4 OH, and further includes hydrogen peroxide (H.sub.2 O.sub.2) and EDA (ethylene diamine).
While such copper and iron solvents have proven to be effective in removing iron oxide and copper from the interior of the secondary side of a nuclear steam generator, they are also capable of promoting new corrosion within the steam generator despite the use of an inhibitor, particularly among the carbon steel and low alloy steel components of the generator. To minimize these corrosive effects, these solvents are typically provided with low concentrations of their active chelate ingredients. Unfortunately, the use of such low concentrations protracts the time it takes for these agents to work, and often necessitates exposing the interior of the secondary side to multiple solvent baths. For example, in one of the most common prior art cleaning methods it is necessary to introduce and to remove an iron solvent twice Within the steam generator, and to introduce and to remove a copper solvent as many as six times. Such multiple solvent baths, along with the various rinse cycles which they necessitate, can cause a single chemical cleaning operation of a steam generator to last one hundred and twenty hours or more. As a utility may typically lose one million dollars in revenues for each day of down time of a nuclear steam generator, it can readily be appreciated that the cost of such a state of the art cleaning operation is quite high, particularly when one considers that the total price of such an operation must also include the cost of the chemicals, the setting up of the recirculation equipment. Further compounding these costs is the fact that the spent iron and copper solvents and rinse solutions that are removed from the radioactive interior of the secondary side of the generator constitute a large volume of radioactive liquid waste which must be disposed of.
Still another shortcoming associated with chemical cleaning operations is the fact that such operations may not be entirely effective in removing tightly ensconced iron oxide and copper impurities from all of the numerous crevice regions within the secondary side of the generator. The applicants have observed that part of this ineffectiveness results from the fact that tightly packed impurities in small spaces do not give the chemical solvent a sufficient opportunity to penetrate and to come into contact with large areas of the surface of such impurities. The applicants have reason to believe that the insoluble fractions of the sludge and other impurities collect as residues at the surfaces of these tightly ensconced deposits during the cleaning operation and hinder the penetration of the chemical cleaning solution beyond the surface of the deposit, thereby stopping or at least significantly retarding the dissolution of the deposits in the crevice regions.
Clearly, what is needed is an improved chemical cleaning method which is faster and more effective than prior art methods. Ideally, such a method would not generate the large volumes of liquid waste associated with the prior art. Finally, it would be desirable if the improved method reduced the probability that the cleaning agents would promote the occurrence of new corrosion within the steam generator.